I've used fresh strawberries a few times and never had an issue with tartness. I guess it depends on the strawberries and whether they're in season though.
I made a vanilla wheat before and it came out great. I made a tincture with 2 vanilla beans and a cup of vodka. Then when it was time...
I remember reading somewhere that SN was doing a split seasonal with Tumbler at the beginning of the season and Flipside at the end. Flipside is great, not that I'd expect anything less from SN. It's my favorite beer of the moment.
I made a Duvel clone once and it took a good 6 months of aging. I tried the beer after 2 weeks and it had a hot alcohol taste. I continued to try it every few weeks and over time the alcohol taste mellowed. I used 22 oz bottles, I'm not sure that kegging would make any difference.
I would try using hop extract. I've never used it myself but I think it would be the easiest way to add more bitterness. Check out this link: http://www.hopunion.com/1020_IsomerizedHopExtract.cfm?p4=open
I agree with urbanmyth. I have made strawberry ale the past three years and my method is to freeze fresh strawberries, then thaw them, then puree with a little vodka, add to secondary and rack the beer on top. Let it sit in secondary for a week before bottling or kegging.